The activation of the receptor TrkB by brain derived neurotrophic factor (BDNF) supports the survival of pure motor neurons in vitro and rescues motor neurons in some in vivo lesion paradigms. In addition, growing motor neurons in the presence of BDNF will induce a state of vulnerability to toxic insults. For example, when motor neurons are grown in a cocktail of trophic factors including BDNF, TrkB antagonism (genetically or pharmacologically) protects against excitotoxic and proteotoxic insults relevant to ALS. The mechanism of this adverse property of BDNF-TrkB signaling is incompletely understood but is known to depend on de novo protein synthesis. TrkB is displayed on the plasma membrane of axons, dendrites and the cell soma. Using Campenot compartment culture system, we find that activation of Trk in the axo- dendritic domain (not the cell soma) renders motor neurons vulnerable to toxic insult. Since axons and dendrites differ dramatically at the cell and molecular biological level it is essential to determine from which compartment the vulnerability signal originates. In Specific aim #1, we will generate chimeric TrkB proteins that target its expression to either the axonal or the dendritic domain. Next we will determine in which compartment TrkB activation leads to vulernability mutant SOD toxicity. The repertoire of mRNAs in the axo-dendritic domain is at least partially distinct from those present in the cell body. A microarray analysis provides us with candidate molecules that might underlie the molecular mechanism of the effect of BDNF. In Specific aim #2, we will manipulate the expression strong candidate molecules and determine if they abrogate the capacity of BDNF to render motor neurons vulnerable to insult. These high-risk-high gain experiments will provide novel insight into the pathogenesis of motor neuron disease. PUBLIC HEALTH RELEVANCE: Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease in which motor neurons die and individuals become progressively weak. In rodent model systems, activation of a cell surface receptor called TrkB can make motor neurons vulnerable to insults relevant to ALS. In this proposal we will determine where on the surface of a motor neuron (e.g., axons versus dendrites) activation of TrkB renders them vulnerable to insult. Next we will study the importance of candidate molecules in axons or dendrites that undergo new synthesis upon activation of TrkB. These studies will bring mechanistic insight into why motor neurons die in ALS.